The present invention relates to a semiconductor device composed of a group III-V nitride semiconductor represented by a general formula (InXAl1xe2x88x92X)YGa1xe2x88x92YN (where 0xe2x89xa6Xxe2x89xa61 and 0xe2x89xa6Yxe2x89xa61 are satisfied) and to a method for fabricating the same.
A group III-V nitride semiconductor such as GaN, AlGaN, InGaN, or InAlGaN, i.e., a so-called gallium nitride-based semiconductor is an important semiconductor for an optical device having a short oscillating wavelength such as a semiconductor laser device outputting, e.g., a blue laser beam. The applications of the gallium nitride-based semiconductor are not limited to the optical device. In recent years, attention has been focused on the gallium nitride-based semiconductor for its high dielectric breakdown field strength, high thermal conductivity, and high electron saturation velocity so that gallium nitride-based semiconductor is considered to be promising also as the material of an RF power device.
In an AlGaN/GaN heterojunction structure composed of aluminum gallium nitride (AlGaN) and gallium nitride (GaN) which are stacked in layers, in particular, electrons are accumulated at a high density in the vicinity of the heterojunction interface between AlGaN and GaN to form a so-called two-dimensional electron gas. The two-dimensional electron gas exhibits a high mobility since it exists spatially separated from a donor impurity used to dope AlGaN. Therefore, the AlGaN/GaN heterojunction structure achieves the effect of reducing a source resistance component when used in a field effect transistor (FET).
Since the distance d from a gate electrode to the two-dimensional gas is normally as small as several tens of nanometers, a ratio Lg/d between a gate length Lg and the distance d, which is termed an aspect ratio, can be held at a large value of 5 to 10 even if the gate length Lg is as small as about 100 nm. Hence, the use of the AlGaN/GaN heterojunction structure offers an advantage of easy fabrication of a FET with a reduced short channel effect and an excellent saturation characteristic.
The electron velocity of a two-dimensional electron in a high-field region of about 1xc3x97105 V/cm in the AlGaN/GaN-based heterojunction structure is double or more the electron velocity thereof in a gallium arsenide-based (GaAs-based) FET which is currently prevalent as an RF transistor, i.e., an AlGaAs/InGaAs heterostructure FET. In addition, the density of electrons accumulated at the heterointerface becomes as high as 1xc3x971013/cm2 when the composition of Al in AlGaN is 0.2 to 0.3, which is about three to five times as high as the density of electrons in the GaAs-based device.
Since the dielectric breakdown field strength of the GaN-based heterostructure FET is about ten times as high as that of the GaAs-based FET, the application of a drain voltage therein which is ideally about ten times a drain voltage applied in the GaAs-based FET becomes possible on the assumption that the GaN-based heterostructure FET and the GaAs-based FET have the same device patterns. This renders the GaN-based heterostructure FET promising as an RF power device capable of generating an output power which is at least 5 times, ideally about 10 times as high as or higher than an output power generated by the GaAs-based power device but it also has numerous problems to be solved.
One of the problems associated with the GaN-based heterostructure FET is a large surface leakage current between the gate and drain thereof A gate electrode composing a GaN-based heterostructure FET is a so-called Schottky gate electrode normally composed of a metal material with a relatively large work function coated directly on a semiconductor. As a metal material composing the Schottky gate electrode, a material having a large work function such as nickel (Ni), palladium (Pd), or platinum (Pt) is used appropriately.
If a current-voltage characteristic between the gate and drain is examined after the metal material is vapor-deposited, an abnormally large leakage current is observed frequently at a reverse voltage and a leakage current value cannot be reduced consistently. The large leakage current significantly increases an idle current component to the power device when a high negative voltage is applied to the gate electrode. As a result, the advantage of the GaN-based heterostructure FET that it can be driven at a high drain voltage cannot be used effectively any more, which presents a critical problem.
The occurrence of such a large gate leakage current may be attributed to a reaction between an oxide film (natural oxide film) formed on a surface of the GaN-based semiconductor and the metal material coated thereon and to a reaction between the surface of the GaN-based semiconductor and the metal material.
To prevent such reactions and thereby reduce the gate leakage current, there has been proposed a so-called MIS (Metal-Insulator-Semiconductor) structure or a MOS (Metal-Oxide-Semiconductor) structure in which an insulating film composed of a silicon nitride (SiN), a silicon dioxide (SiO2), or the like is deposited on the surface of the GaN-based semiconductor and a gate electrode is formed on the deposited insulating film.
However, since the oxide film mentioned above exists at the interface between the gate insulating film and the GaN-based semiconductor or a trap is easily introduced into a surface of the semiconductor by a surface treatment performed in the fabrication process, a GaN-based FET using the MIS structure or MOS structure described above is not necessarily stable because of the current-voltage characteristic thereof which varies depending on the operating frequency.
As a result of making various examinations, the present inventors have found that a FET having a MOS structure obtained by directly thermally oxidizing a GaN-based semiconductor to form a thermal oxide film on the surface of the semiconductor, not by depositing an insulating film on the GaN-based semiconductor, and forming a gate electrode on the formed thermal oxide film has a relatively stable operating characteristic. This indicates that the number of traps formed in the interface between the thermal oxide film and the GaN-based semiconductor is relatively small so that such a MOS structure is promising in terms of stabilizing the operation of the FET and reducing a gate leakage current.
The present inventors have also found that, if the thickness of the thermal oxide film is relatively small in the MOS structure, the effect of reducing the gate leakage current is not sufficient and therefore the thickness of the thermal oxide film should be increased to a certain extent. The presumed reasons for this are as follows: Since a thermal oxide film formed by oxidizing a semiconductor composed of AlGaN is composed of gallium oxide (Ga2O3) and aluminum oxide (Al2O3) and contains, as a main component, gallium oxide having a relatively small energy gap of about 4.2 eV, the energy barrier thereof against electrons is not sufficiently high compared with the energy barrier of silicon dioxide having a large energy gap of 10 eV In addition, the thermal oxide film composed of polycrystals of gallium oxide, aluminum oxide, and the like contains numerous grain boundaries in the polycrystals so that a leakage current flows through the grain boundaries.
Increasing the thickness of the gate oxide film to reduce the leakage current flowing through the grain boundaries reduces the transconductance of the FET disadvantageously so that the problem of a reduction in the performance of the FET occurs.
On the other hand, it has been reported that oxygen vacancies are formed readily at a high temperature in a MOS structure using a thermal oxide film formed by oxidizing a GaN-based semiconductor. Since the vacancies may function as an n-type impurity and change the conductivity of the thermal oxide film or the thermal oxide film may react with a metal material formed thereon, the long-term reliability cannot be obtained.
The present invention has been achieved in view of the foregoing conventional problems and it is therefore an object of the present invention to reduce a gate leakage current in a FET using a GaN-based semiconductor and improve the long-term reliability thereof.
To attain the object, a semiconductor device according to the present invention is composed of a GaN-based semiconductor and has a gate insulating film composed of an oxide film formed by oxidizing the GaN-based semiconductor and a discrete insulating film formed on the oxide layer.
Specifically, a semiconductor device according to the present invention comprises: a first semiconductor layer composed of a group III-V nitride; an oxide film formed by oxidizing a second semiconductor layer composed of a group III-V nitride to be located on a gate electrode formation region of the first semiconductor layer; an insulating film formed on the oxide film to have a composition different from a composition of the oxide film; and a gate electrode formed on the insulating film.
In the semiconductor device according to the present invention, the two-layer gate insulating film consisting of the oxide film formed by oxidizing the second semiconductor layer composed of the group III-V nitride and the insulating film formed on the oxide film to have a composition different from that of the oxide film is formed between the gate electrode and the first semiconductor layer composed of the group III-V nitride. Since the oxide film has been formed by oxidizing the semiconductor composed of the III-V nitride, the interface with the first semiconductor layer is in an excellent state with traps at a low density. This considerably reduces a gate leakage current and thereby significantly improves the output characteristic. In addition, the insulating film having a composition different from that of the oxide film and interposed between the oxide film and the gate electrode keeps the oxide film from direct contact with the gate electrode. This prevents a reaction between the respective materials composing the oxide film and the gate electrode and thereby improves the long-term reliability of the oxide film formed by oxidizing the semiconductor composed of the group III-V nitride.
In the semiconductor device according to the present invention, an energy gap of a material composing the insulating film is preferably larger than an energy gap of a gallium oxide.
In the semiconductor device according to the present invention, a thickness of the insulating film is preferably equal to or smaller than a thickness of the oxide film.
A first method for fabricating a semiconductor device according to the present invention comprises the steps of: selectively thermally oxidizing a gate electrode formation region of a semiconductor layer composed of a group III-V nitride to form an oxide film formed by oxidizing at least an upper portion of the semiconductor layer; forming, on the oxide film, an insulating film having a composition different from a composition of the oxide film; and forming a gate electrode on the insulating film.
Since the first method for fabricating a semiconductor device forms the gate insulating film from the oxide film formed by oxidizing at least the upper portion of the semiconductor layer composed of the group III-V nitride and from the insulating film formed on the oxide film to have a composition different from that of the oxide film, the semiconductor device according to the present invention can be obtained reliably.
In the first method for fabricating a semiconductor device according to the present invention, the insulating film is preferably composed of a silicon oxide.
In the first method for fabricating a semiconductor device according to the present invention, a thickness of the insulating film is preferably equal to or smaller than a thickness of the oxide film.
A second method for fabricating a semiconductor device according to the present invention comprises the steps of: forming a semiconductor layer composed of a group III-V nitride; forming, on the semiconductor layer, a thin film having a composition different from a composition of the semiconductor layer; selectively thermally oxidizing a gate electrode formation region of each of the semiconductor layer and the thin film to form a first oxide film formed by oxidizing at least an upper portion of the semiconductor layer and a second oxide film formed by oxidizing the thin film; and forming a gate electrode on the second oxide film.
Since the second method for fabricating a semiconductor device forms the gate insulating film from the first oxide film formed by oxidizing at least the upper portion of the semiconductor layer composed of the group III-V nitride and from the second oxide film formed by oxidizing the thin film, the semiconductor device according to the present invention can be obtained reliably.
In the second method for fabricating a semiconductor device according to the present invention, the thin film is preferably composed of silicon.
In the second method for fabricating a semiconductor device according to the present invention, a thickness of the second oxide film is preferably equal to or smaller than a thickness of the first oxide film.
A third method for fabricating a semiconductor device according to the present invention comprises the steps of: forming a semiconductor layer composed of a group III-V nitride; forming an insulating film on the semiconductor layer; selectively thermally oxidizing a gate electrode formation region of the semiconductor layer to form an oxide film formed by oxidizing at least an upper portion of the semiconductor layer; and forming a gate electrode on the insulating film.
Since the third method for fabricating a semiconductor device forms the gate insulating film from the oxide film formed by oxidizing at least the upper portion of the semiconductor layer composed of the group III-V nitride and from the insulating film formed preliminarily on the oxide film, the semiconductor device according to the present invention can be obtained reliably.
In the third method for fabricating a semiconductor device according to the present invention, the insulating film is preferably composed of a silicon oxide.
In the third method for fabricating a semiconductor device according to the present invention, a thickness of the insulating film is preferably equal to or smaller than a thickness of the oxide film.